Closed cycle Stirling cryogenic coolers (refrigerators) are well known and used widely to maintain various electronic devices and systems at cryogenic temperatures. Infrared sensors, demagnetization devices, infrared interferometers, cryogenically cooled optics, filters and low-noise cryogenic electronic devices are representative electronic components requiring cryogenic cooling. Such refrigerators typically comprise expanders interconnected to a pressure wave generator using a transfer line or conduit.
The expander portion of such a cooler typically comprises a movable displacer-regenerator which is supported inside a cold finger housing using an appropriate arrangement of pneumatic and mechanical springs forming a vibratory system with properties to resonate with the desired stroke and optimal phase lag relative to the pressure oscillation arriving from the pressure wave generator, thus shuttling the working agent (typically helium) back and forth from the cold side to the hot side of the cold finger portion of the expander, as needed for producing a useful cooling effect, as explained, for example, in G. Walker, “Cryogenic Coolers, Part 2—Applications”, Plenum Press, New York, 1983.
The purpose of a pressure wave generator, therefore, is to produce low frequency oscillatory pressure pulses and volumetric changes of a working agent inside a hot space of the expander which are used for actuating the movable regenerator-displacer assembly and for supplying pressurised gas to the expansion space of the cold finger.
In a piston pressure wave generator, the low frequency pressure waves are produced by a piston reciprocating inside a tightly matched cylinder sleeve, where the separation of the compression space, for the sake of reliability, normally relies on a dynamic clearance seal. The piston is driven by a rotary drive through a crank-slider arrangement or directly using a linear voice coil motor, such as of a “moving magnet” or “moving coil” design, as described for example in U.S. Pat. Nos. 5,596,875, 4,365,982 and 6,094,912.
The known disadvantages of the above pressure wave generators include large bulk (size and weight), and typically low performance of electromechanical driving and gas compression. Furthermore, such a pressure wave generator cannot deliver high compression ratios, due to the inherently large dead volumes and parasitic back-flows through the dynamic clearance seals. In addition, the life span is limited because of friction and wear. Other disadvantages are the noise and vibration produced by the imbalance, and micro-collisions occurring between the moving components.
Also known are membrane pressure wave generators, comprising a linear motor of voice coil, electromagnetic or solenoid type driving a compliant diaphragm which is attached to a rigid compression chamber, as described for example in U.S. Pat. No. 5,645,407. The known disadvantages of such an approach are low reliability, low performance of electromechanical driving, mechanical complexity and high bulk and inherently large dead volume that prevent developing high compression ratios.
Another known generator is an acoustic pressure wave generator (sonic compressor), in which large pressure oscillations are obtained by acoustic resonant amplification of relatively weak acoustic waves produced by a loudspeaker inside an optimally shaped gas chamber, as described for example in U.S. Pat. Nos. 5,231,337, 5,174,130, 5,263,341 and 5,020,977, amongst others. The disadvantage of such an approach is that a relatively bulky gas chamber is needed to support a desirable low frequency resonance. Further disadvantages are low performance of electromechanical driving and the sensitivity of the low frequency resonance to the ambient temperature.
Also known are low frequency pressure wave generators that operate thermoacoustically, as described for example in U.S. Pat. Nos. 4,953,366 and 5,901,556. Such a device typically comprises an internally heated gas cavity with an attached resonant tube or cavity. Development of low frequency intensive gas compression waves relies on thermoacoustic instability in a Helmholtz resonator with non-uniformly heated walls, and manifests itself in the form of self-sustained oscillations synchronized with the natural acoustic resonance. Disadvantage of such an approach are extremely low performance and a relatively bulky gas chamber is needed to support a desirable low frequency resonance.
Pumps are also known for transporting gases, vapours and liquids having a tubular shape and accommodating electrodes electrically connected to a pulsed electrical power supply, and having check valves means mounted in the inlet and outlet sections of the pump. Such pumps are described for example in U.S. Pat. Nos. 2,050,391 and 3,266,438. In such devices, in response to a low frequency sequence of rectangular high voltage pulses, the low frequency electrical discharges in a working fluid produce a low frequency electrical arcing leading to a generation of a low frequency thermal plasma resulting in a the low frequency sharp expansion of the working fluid, which is arranged to flow in the desired direction using the check valve means.
The disadvantage of such an approach, as applied to pressure wave generators of a Stirling cryogenic cooler, is poor conversion of electrical energy into the energy of the compressed working fluid through the essential overheating (typically up to 10,000K) occurring inside the core of discharge arc. The associated heat flux generated by this mechanism is then sunk to the environment and rejected from the thermodynamic cycle. Further disadvantages of this approach are intrinsically diminished performance of the Stirling thermodynamic cycle due to the increase of the reject temperature, increase of the heat loading through heat conductivity and radiation, and development of debris contamination the cooler interior.